A conventional construction of a first-described spindle motor for magnetic disk devices (an outer rotor-type DC brushless motor) is shown in FIG. 2 as including a stator 1; a magnetic disk-mounting hub 2 axially supported on a frame 3 of the stator 1 via a bearing 4; a rotor magnet 5 (filed permanent magnet) fitted on the inner side of the outer circumference of the hub 2 in a position opposite to a magnetic pole of the stator 1; a clamp 6 to secure a magnetic disk 7 mounted on the hub 2; and a screw 8 to secure the clamp 6. The hub 2 is made of aluminum, which is also used for the substrate of the magnetic disk 7, and has a bowl-like yoke 2a protruding from its base in the radial direction, to which the magnetic disk 7 is mounted using the flat top of the yoke 2a as a seating face. A magnetic head 9, supported on a support member 10, is provided to write to and read information items or data from the magnetic disk.
A magnetic disk device of the construction shown in FIG. 2 is commonly known. In such a conventional device, the magnetic disk 7 is mounted on a spindle motor to rotate the disk. Furthermore, the magnetic head 9 is scanned in this condition along tracks on the information recording face of the magnetic disk 7, and the magnetic head 9 is moved in the radial direction by a head carrier (not shown in the figure) to access a selected track and write in and read out information items.
The spindle motor with a conventional construction as described above has a number of problems. For example, when the magnetic disk 7 is mounted on the spindle motor, the inner circumferential area of the magnetic disk 7 is superposed with the top of the yoke 2a in the hub 2. Therefore, the area that the magnetic head 9 can access, or the area where information can be written in or read out, is limited of its own accord to the area outside the circumference of the yoke 2a. Moreover, the specification for the spindle motor is determined from the motor starting torque, rated output, and measures to prevent off-tracks caused by shaft run-out, and the reduction of the outline dimensions of the motor from the design aspect is limited. For this reason, motors with a conventional construction are almost incapable of coping with magnetic disks with small diameters (1.8 inches or less) for example.
In addition, when the magnetic head 9 moves toward the inner circumference of the magnetic disk 7 approaching the hub 2 of the spindle motor, a motor with a conventional construction may suffer a magnetic flux leak from the rotor magnet 5 or the stator 1 which may cause noise effecting the magnetic head 9, thereby leading to a risk of errors in writing in and reading out information. Means to prevent influence of the magnetic flux leaking from the spindle motor include enclosing the motor by using a magnetic body such as iron to magnetically shield the entire hub from the magnetic disk. This construction, however, may cause another problem since, when ambient temperatures changes, the difference in the thermal expansion of the magnetic disk substrate (usually made of aluminum) and the hub may cause mechanical distortion in the magnetic disk.
The present invention has been made in the light of the above problems, and is intended to provide a spindle motor for magnetic disk devices which can eliminate the above-mentioned problems and is capable of coping with a small magnetic disk, in which magnetic flux leakage from the motor does not adversely affect the magnetic head.